The present invention relates to a circuit arrangement for measuring decelerations of a vehicle which, caused by a braking operation occurring before a crash, lead to a change in the seat position of vehicle occupants, wherein an analog/digital converter converts the analog output voltage of an acceleration sensor for an ensuing signal evaluation into a digital measurement signal.
From the Proceedings of the 3rd International Symposium on Sophisticated Occupant Safety Systems, Airbag 2000, 1996, November 26/27, Karlsruhe, Germany, pages 16-1 through 15-20, it is known, using so-called precrash sensors, with which the distance or relative speed of the vehicle relative to an obstacle can be ascertained on the basis of radar, infrared radiation, or ultrasound, to make a prediction whether a crash of the vehicle and an obstacle is imminent. This reference also describes an interior sensor which detects the positions of a vehicle occupant on his seat by infrared radiation and ultrasound. Depending on the position of the vehicle cccupants, the tripping of an airbag, or in other words the intensity of its inflation, is controlled such that the occupant is given optimal protection in a vehicle crash. If the vehicle occupant is in what is called an out-of-position location in which the tripping of an airbag could be too dangerous to the occupant, the airbag is deactivated.
Experience teaches that the driver of a vehicle, before a natural crash occurs, initiates a braking operation. This braking operation, also known as precrash braking, means that above all an occupant on the front passenger seat is shifted forward toward the front passenger airbag. From the deceleration that the vehicle undergoes as a result of such a braking operation before a crash, in cooperation with the interior sensor, the positional shift of the occupant can be detected in good time before the ensuing crash, so that an activation or deactivation of the airbag can be initiated accordingly.
The object of the invention is now to disclose a circuit arrangement for measuring decelerations of a vehicle thatxe2x80x94caused by a braking operation occurring before a crashxe2x80x94lead to a change in the seat position of vehicle occupants, which is feasible with the simplest possible switching means.
This object is attained with the characteristics of claim in that the output signal of an acceleration sensor, once it has been converted by an analog/digital converter into a digital measurement signal, is evaluated. As the acceleration sensor, a sensor design to detect high decelerations that occur in a crash is used. An amplifier is provided, which amplifies the sensor output voltage, ascribed to low decelerations, so that the digital measurement signal derived from the amplified output voltage by the analog/digital converter has an adequately high resolution of the decelerations.
According to the invention, to detect a braking operation occurring before a crash, an acceleration sensor that is present in the vehicle anyway is accordingly used; it is actually intended to measure high accelerations, occurring in a natural crash, in the longitudinal direction of the vehicle that are up to 35 times the acceleration due to gravity, or g. The invention makes it possible for a sensor, intended intrinsically to measure very accelerations, to be utilized as well to detect low decelerations (0.1 g to 1 g), of the kind that occur in precrash braking operations, and the measurement signal has adequately high resolution for the low decelerations.
Advantageous refinements of the invention are defined by the dependent claims. Accordingly, switching means can be connected to the amplifier, which limit or convert the rise of the amplifier output voltage to the voltage range that the analog/digital converter is capable of converting. The amplifier can for instance be an operational amplifier, at whose output a transistor is connected as a collector circuit, which sets an upper limit to the rise of the output voltage of the operational amplifier to the voltage range that the analog/digital converter is capable of converting and extends it downward to near the ground potential. An inexpensive bipolar operational amplifier can thus be used, which has relatively high output saturation voltages, so that even so, high resolution of the acceleration measurement signal can be attained.